If platform operators openly stated "anything you develop can and will be used against you - by us" then no one in their right minds would develop on those platforms.
There is a difference between "using something against you" and a platform keeping their options open.
But, to quote Tim Bray from 2003:
They own the ground you’re building on, and if they decide they don’t like you, or they can do something better with the ground, you’re toast. They can ship their own product and give it away till you go bust, then start charging for it; and use secret APIs you can’t see; and they can break the published APIs you use. All of these things have historically been done by platform vendors.
My point is that no platform has ever behaved differently to how Facebook are now, and Facebook are at least offering money to companies they are planning on competing with.
People will continue to develop for these platforms, though, because they offer the incredible lure of money - they are were the users are, and where the users are the money is too.